BEIRUT — A U.S.-backed force of Kurds and Arabs advanced toward an important Islamic State transit town in Syria on Wednesday, brushing aside Turkish opposition to the involvement of Kurds in operations to recapture the strategically vital area.

U.S. commandos are accompanying the Kurdish and Arab Syrian Democratic Forces (SDF) as they push north toward Manbij, backed by intense U.S. airstrikes, as part of an offensive aimed at recapturing the town in Aleppo province, said Col. Chris Garver, a U.S. military spokesman.

Manbij lies on the main route used by foreign fighters traveling across the Turkish border to join the Islamic State in the northern Syrian city of Raqqa, the de facto capital of the extremist group’s self-proclaimed caliphate. Capturing Manbij would sever the vital supply route, cut off militant fighters and further squeeze the Islamic State in Raqqa, Garver said.

But the offensive also risks incurring the wrath of Turkey, which reiterated this week its opposition to using the SDF to take control of the predominantly Arab area. The SDF is dominated by the Kurdish People’s Protection Units, or YPG, which has ties to the militant Turkish Kurdistan Workers’ Party, or PKK. The PKK is designated a terrorist organization by both the United States and Turkey.

An American fighter, battling alongside the Syrian Democratic Forces, holds a U.S. flag as he stands with SDF members in the northern province of Raqqa, Syria, May 27, 2016. (Rodi Said/Reuters)

The Kurdish organization has declared an autonomous region in the part of northeastern Syria it already controls, and Turkey fears that its continued advance westward into Arab areas of Syria will further expand the Kurdish region emerging along the Turkish border and encourage Kurdish separatist sentiments inside Turkey.

Pentagon officials said that the SDF fighters advancing on Manbij are predominantly Arab and that the YPG has agreed not to retain control of the town after it is captured but rather to hand over its administration to local Arabs. Garver put the percentage of Arabs in the advancing force at 80 to 90 percent.

But SDF units fighting elsewhere are overwhelmingly Kurdish. One U.S. official, who was not authorized to publicly discuss the issue, said that overall, “Kurds are the main fighting force” in the SDF. “We shouldn’t be under any illusions about that.”

Despite Pentagon claims that Turkey has consented to the operation, the Turks are “not happy” about it, the official added. Turkish officials did not respond to requests for comment.

The offensive signals the abandonment, at least for now, of a long-standing U.S.-Turkish plan to coordinate on efforts to reconquer the Islamic State-held pocket of territory in Syria’s Aleppo province, known as the Manbij pocket, using vetted Free Syrian Army rebel groups. Those efforts have stalled since the Islamic State repelled a rebel offensive in March aimed at recapturing areas of the province that border Turkey. The Islamic State subsequently has not only pushed the rebels back from the areas they have taken but also has captured new territories that were formerly under rebel control. These gains have left the U.S.- and Turkish-backed groups battling for their survival and in no position to launch any new offensives against the al-Qaeda offshoot, which is also known as ISIS and ISIL.

Instead, the United States has decided to back the Kurdish-led assault on the Manbij area. “We have bitten the bullet on the Kurds,” said the U.S. official.

The Manbij operation comes as part of a broader effort to squeeze the Islamic State in Raqqa. It began last week with a push by the SDF to capture villages in the mostly desert area immediately north of the city. Those forces have advanced several miles, and the operation is ongoing, Garver said.

The Raqqa offensive also claimed its first U.S. casualty over the weekend. One of the 300 U.S. Special Operations forces­ working alongside Kurdish and Arab fighters in northeastern Syria was injured north of Raqqa over the weekend, the Pentagon said Tuesday.

DeYoung reported from Washington.

Clarification:This story has been updated to clarify that Col. Chris Garver did not describe the force advancing on Manbij as Kurdish-led.